The present invention relates to a DLL (Delay Locked Loop) circuit and, more particularly, to a DLL circuit having the function of reducing intensity of electromagnetic radiation noise.
An LSI (Large Scale Integration) mounted on an electronic device emits electromagnetic waves in response to power consumption and to operation frequency. Electromagnetic waves exert an adverse influence such as electromagnetic radiation noise on an entire electronic device. The main cause of the electromagnetic radiation noise is electromagnetic waves generated from a DLL circuit in the LSI and other circuits using an output signal (DLL output signal) of the DLL circuit. In particular, strong electromagnetic waves are generated from the other circuits using the DLL output signal.
On the other hand, as electronic devices are sophisticated and the integration technique has improved in recent years, the power consumption, operation frequency, packaging density, and the number of LSIs mounted on an electronic device have been increasing. As a result, the intensity of electromagnetic waves (electromagnetic radiation intensity) emitted from the LSI increases, and the influence of electromagnetic radiation noise exerted on the electronic device is becoming unignorable.
An ordinary LSI has a DLL circuit. When the DLL circuit is in a locked state (that is, the DLL circuit and the LSI having the DLL circuit are in an operating state), the frequency of a DLL output signal of the DLL circuit has a constant value (DLL locked frequency). As a result, the electromagnetic radiation intensity at the DLL lock frequency becomes very high.
A DLL circuit suppressing minimum delay time (minimum slew rate) of delay time which can be adjusted in a wide range is known (Japanese Patent Application Laid-open Publication No. 2004-260663). However, the Japanese Patent Application Laid-open Publication No. 2004-260663 does not disclose means for reducing the electromagnetic radiation noise caused by the DLL circuit.
Therefore, the electromagnetic radiation intensity of the conventional DLL circuit and that of an LSI having the DLL circuit is high. There is a problem such that an electronic device having the LSI is adversely influenced by the electromagnetic radiation noise.
Furthermore, a peak of the electromagnetic radiation intensity of the conventional DLL circuit and that of the LSI having the DLL circuit appears at particular operation frequency. Since this peak is very high, the electronic device having this LSI is adversely influenced by the electromagnetic radiation generated from this LSI. Therefore, the LSI having DLL circuit is required to have a function to prevent EMI (Electro Magnetic Interference).
However, the function to prevent EMI depends on configurations of the electronic device. Therefore, if the function to prevent EMI having scalability, for example the function for shifting distribution of frequency and for controlling shape of distribution of frequency, is not used, a various restrictions is imposed users which use the LSI.